Welcome to the forum!
Since it was in poor conditions prior to its adoption, because you have it in better conditions now, it probably "colored up", meaning you can see its true colors now.
Make sure you have some hidey-holes for the betta so she can get away from fish if they start acting aggressively toward her...you already have a heater, so I assume you have it set at tropical temperatures (76ºF-80ºF)?
She may have fin rot if her top fin is missing...check her for discoloration around the fins that appear to be brown or ragged (even with white fuzz, it could be fungal). If she does have this, remove her from the tank and put her in a quarantine tank. The bacteria / fungus that causes fin rot is contagious and can affect your other fish. If she does have fin rot, please post back here and we'll do our best to help you.
Other than that, it sounds like you know what you're doing. Great job on rescuing her, she's probably a lot happier now.